The invention relates generally to mechanical probe tips such as those used in atomic force microscopy. In particular, the invention relates to a composite probe tip having a surface coating overlying a base member.
Atomic force microscopes (AFMs) have been recently developed for mechanically profiling small features, for example, determining critical dimensions (CDs) of via holes in semiconductor integrated circuits. Such holes have depths of about 1 xcexcm and widths which are being pushed to 180 nm and below. An exceedingly fine probe tip is disposed on the ends of cantilever. In the pixel mode of operation, the probe tip is successively positioned at points on a line above and traversing the feature being probed. The cantilever lowers the probe tip until it encounters the surface, and both the horizontal position and the vertical position at which the probe meets the surface are recorded. A series of such measurements provides the desired microscopic profile. An example of such an atomic force microscope is the Stylus Nanoprobe SNP available from Surface/Interface, Inc. of Sunnyvale, Calif. It employs technology similar to the rocking balanced beam probed disclosed by Griffith et al. in U.S. Pat. No. 5,307,693 and by Bryson et al. in U.S. Pat. No. 5,756,887. Mitchell et al. describe more details of the SNP type of AFM in U.S. patent application Ser. No. 09/354,528, filed Jul. 15, 1999 and incorporated herein by reference in its entirety.
This and other types of AFMs have control and sensing elements more than adequate for the degree of precision required to profile a 180 nmxc3x971 xcexcm hole, as required for many present day integrated circuits. However, the probe tip presents a challenge for profiling the highly anisotropic holes desired in semiconductor fabrication as well as for other uses such as measuring DNA strands and the like. The probe tip needs to be long, narrow, and stiff. Its length needs to at least equal the depth of the hole being probed, and its width throughout this length needs to be less than the width of the hole. A fairly stiff probe tip reduces the biasing introduced by probe tips being deflected by a sloping surface.
One popular type of probe tip is a conically shaped silica tip. It is shaped by a method similar to that used for fabricate a cylindrically shaped probe tip, as described by Marchman in U.S. Pat. No. 5,395,741 and 5,480,049 and by Filas and Marchman in U.S. Pat. No. 5,703,979. For the conically shaped probe tip, a thin silica fiber having a diameter of, for example, 125xcexcm, is lowered through the interface of a thin layer of oil overlying a liquid etchant such as hydrofluoric acid and left there for a sufficient time that a generally conical tip is formed at the fiber end. The etching time is longer than that used for forming Marchman""s cylindrical tips. Tips made from silica (SiO2) or other glasses having nearly this composition are usually referred to as being composed of quartz. The tip manufacturing described by Marchman is relatively straightforward, and the larger fiber away from the tip provides good mechanical support for the small tip to be mounted on a beam or cantilever of the profilometer. However, it is difficult to obtain the more desirable cylindrical probe tip of his method. Furthermore, silica is relatively soft so that its lifetime is limited because during use it is being repetitively forced against a relatively hard substrate. Also, silica has a relatively low value of 73 GPa for its Young""s modulus, which determines the stiffness of the probe. Very narrow probes tend to flex when they encounter sloping surfaces. Flexing also occurs when very narrow probes are attracted to sidewalls due to electrostatic charging or capillary action in a humid environment. Such flexing should be minimized to enhance the resolution of the AFM or other instrument using the probe.
Mitchell et al. in the above cited patent application disclose a micromachined microprobe in which the probe tip is fabricated using standard deposition and etching processes common in the integrated circuit industry. They favor depositing a thin layer of silicon nitride on a silicon substrate, photolithographically defining the probe width into the silicon nitride layer, and etching away part of the substrate underlying the silicon nitride probe tip. Silicon nitride has a Young""s modulus of 270 GPa so it provides a much stiffer probe.
However, the photolithographic resolution required to define the very narrow width of the micromachined microprobe of Mitchell et al. inevitably needs to be slightly better than that being used in the most advanced integrated circuits being developed and tested by the microprobe. As a result, Mitchell et al. suggest fabricating a micromachined microprobe with a significantly wider width than that desired and then reducing its width with focused ion beam (FIB) milling. FIB milling is capable of resolutions of 7 nm, which is satisfactory for most present probe requirements. FIB tools are commercially available.
Although the micromachined microprobe shows promise, it requires development of a new technology to replace the quartz probe tips of Marchman, which are widely accepted and inexpensive but subject to excessive wear and flexing.
FIB milling could be also used to mill very small probe tips from larger bodies of sapphire or diamond. However, FIB milling while effective at milling small areas becomes tedious and expensive when applied to removing large amounts of material.
Accordingly, a probe tip is desired which is both inexpensive but is more resistant to wear than is quartz. Further, a probe tip is desired which is stiffer than one composed of quartz.
The invention may be summarized as a probe tip formed from a generally conical quartz tip which is isotropically coated with a thin layer of wear resistant material. The coating thickness may range between 100 nm to 10 xcexcm, preferably between 500 nm to 5 xcexcm, and most preferably between 1 and 2 xcexcm. The wear resistant material may be silicon nitride, alumina, diamond, or carbides such as silicon carbide among other possibilities. The wear resistant material is then milled, for example, by focused ion milling (FIB), to form a probe tip.
The machined probe tip may have a cylindrical or square cross section, among other possibilities.